Abstract

Flux pinning effect is related to the strong influence by the presence of defects that act as pinning centers. We present transport experiments that investigate the pinning properties in a spacing-graded array of pinning sites. We have found the asymmetric pinning by arrays of defects with modulated gradient spatial distributions on niobium film. One can clearly see the asymmetric pinning effect for the vortex motion. The dc voltage depends on the amplitude of the ac current and that there is a sharp maximum at matching fields. This curve, however, has an interesting sign reversal phenomenon below the first matching field, which will be investigated more in the future. The dc voltage as a function of dc and ac applied current gives evidence that the artificial arrays of gradient pinning site density induce ratchet effect. This effect can be used to control the vortex motion, which will be important for the application of vortex based on electronic devices.